Hall-Heroult electrolytic cells conventionally employ conductive carbon electrodes. In the Hall-Heroult process, a current is passed between a carbon anode and a carbon cahode in a cryolitic electrolyte containing dissolved alumina. Aluminum metal is reduced from the alumina, and the carbon anode is consumed in the process.
The overall reaction, in its simplest form, is represented as follows: ##STR1## Approximately 0.33 pounds of carbon are consumed for every pound of aluminum produced, providing a typical useful anode life of two to three weeks.
Aluminum can also be produced by reduction of alumina using the following electrolytic reaction. EQU 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2
The anode liberates oxygen from the alumina, and aluminum metal is produced at the cathode. When employing such a process, anodes constructed of a material other than carbon are used. The anode is not consumed as the chemical composition of the anode does not enter into the electrolysis reaction. Such anodes would theoretically have a life limited only by corrosion due to the cryolite electrolyte and electrochemical degradation mechanisms. It is anticipated that the life of such anodes could be extended to several months or even a year or more as compared to the 2 to 3 week life of a carbon anode which is consumed in the electrolytic reduction reaction. However, most all of these anodes rapidly or catastrophically degrade under normal cell operating conditions.
We have discovered new methods and an apparatus for minimizing corrosion of anodes used in the Hall-Heroult process for production of aluminum.